Gas temperature is one of the main factors in determining the output power obtainable from a gas laser. Therefore, management of the heat generated during laser operation is critical to overall performance. In sealed off, RF excited carbon dioxide lasers, for example, electrodes are used to excite the gas plasma. These electrodes are traditionally made from metal such as aluminum and are spaced apart so as to form a gap therebetween in which the gas is excited to form the plasma. The metal electrodes are consequently in contact with the plasma and conduct heat from the plasma in an efficient manner. This heat must then be removed from the electrodes in order to maintain a desired operating temperature.
Cooling the electrodes can be accomplished by several methods. One method is to directly pump liquid through the electrodes. Another method is to conduct the heat from the electrodes through the walls of the tube and then dissipate the heat outside of the tube, for example by air cooling or liquid cooling. Each method has its advantages and disadvantages.
Thus, liquid cooling of the electrodes using the first method is the most efficient method, but it requires complicated seals and additional passageways to direct the liquid into and out of the sealed tube. The electrodes tend to be more complicated as well, in order to accommodate the liquid cooling.
Conducting the heat from the electrodes through the walls of the tube using the second method is usually less complicated and expensive. However, this approach requires that the heat be transferred through a layer of dielectric material surrounding the electrodes, because the electrodes are under electric potential and cannot therefore be in direct contact with the walls of the tube. This reduces the heat transfer efficiency.
Because of its lesser cost, the second method is the preferred approach for lower cost lasers.
Low power lasers can employ a small gap between the electrodes and the walls of the tubes. This gap, which is occupied by the laser gas mixture, serves the same function as a layer of dielectric material in electrically insulating the electrodes, while the gas transfers the heat across the gap. However, as power increases, the heat load increases and this method becomes impractical due to the poor thermal conductivity of gases.
For high power lasers, solid dielectric materials with good thermal conductivity, such as alumina ceramic, are employed. The dielectric material is sandwiched between each electrode and the walls of the tube. One example of a conventional laser with such a heat transfer system is described in U.S. Pat. No. 6,198,758, the disclosure of which is incorporated herein by reference.
An improved construction is described in U.S. Pat. No. 5,661,746, which illustrates in FIGS. 13-15 therein the use of anodized aluminum (aluminum oxide coated) spacers to insulate the electrodes from the walls of the tube while maintaining good thermal conductivity.
However, there have been a number of difficulties that need to be overcome when implementing a design using solid dielectric insulators to conduct heat from the electrodes.
First, all of the components, including the electrodes, tube walls and the dielectric layers, have had smooth, flat surfaces in order to maximize contact between the components for optimal heat transfer. Tubes for sealed gas lasers are usually extruded, and extrusions tend to have a certain amount of surface distortion, making it difficult to obtain flat internal surfaces. Electrodes are also sometimes extruded, leading to the same problem. Machining the electrodes to obtain flat surfaces is expensive, and machining the inside surfaces of the walls of the tubes is very difficult.
Because the tube walls cannot easily be machined flat, they must be made thin, so that they will deform when the tube is compressed against the assembly of dielectric material and electrodes in order to optimize the contact area. When all or most of the walls are thin, the tube is mechanically weak, which can cause the resonator optical elements to come out of alignment if the resonator optical elements are attached directly to the tube.
In addition, dielectric materials tend to be brittle and are liable to crack when deformed. As a result, in order to compensate for the lack of flat, smooth surfaces and to prevent the dielectric material from cracking under pressure, many smaller pieces of dielectric material have been used instead of fewer larger pieces, but this leads to complexity in assembly and added cost.
One further problem arises from the fact that the electrodes and dielectrics must be held in place in the tube to prevent the electrodes from making contact with the walls of the tube. Pockets can be machined into the electrodes to contain the dielectric material, but it is difficult to machine pockets on the inner walls of the tube. Consequently, other methods must be employed to prevent the electrodes from shifting laterally and touching the walls.
In an attempt to address some of these problems, it has been proposed to make the laser tube that holds the electrodes deformable. Two designs for deformable laser tubes are known, but while each one addresses the problem of providing surface contact for satisfactory heat transfer, each still has drawbacks that need to be overcome.
The first design is disclosed in U.S. Pat. No. 4,787,090, which proposes an extruded tube (11 in FIG. 1 of the patent) with a U-shaped structure (20, 21 and 22 in FIG. 1) on one wall connected to the rest of the tube by thin walls (18 and 19 in FIG. 1). The thin walls allow the tube to deform, bringing the U-shaped structure into contact with the electrode assembly. To achieve this, the electrode assembly is inserted into the tube and then a separate part or top (31 in FIG. 2 of the patent) is bolted to the tube in such a way that it applies force to the deformable thin walls of the tube, compressing the electrode assembly to provide surface contact for heat transfer.
This design has a number of drawbacks. First, the design is complicated by the fact that the provision for compressing the tube is not built into the tube itself, but rather requires the separate top 31 to be attached to the tube.
Another drawback is that the electrode assembly is compressed by vertical members (20 and 21 in FIG. 1), which contact only a small portion of one face of the electrode assembly. The intermediate portion of the tube wall (22 in FIG. 1) does not contact the electrode assembly. However, on the opposite face of the electrode assembly, the tube wall (15 in FIG. 1) contacts the entire facing surface of the electrode assembly. This results in asymmetrical cooling that can cause the tube to deform, which can affect beam quality and resonator alignment if the optical elements are mounted directly to the tube.
The second design for a deformable tube is described in U.S. Pat. No. 6,195,379, which proposes a rectangular extruded tube (44 in FIG. 10 of the patent) with thin walls squeezing the electrode assembly and providing the compressive force for surface contact to facilitate heat transfer.
This second design also suffers from drawbacks. The extruded tube does not provide a means for capturing the electrode assembly and preventing it from shifting laterally other than the friction from the compressive force. Friction is not a reliable means to laterally locate the electrodes in the tube and keep them aligned with the optics, or to prevent them from contacting the walls of the tube and causing an electrical short circuit.
This design also relies on the elastic deformation, temper and material strength of the tube to compress the electrode assembly. This restricts the materials from which the tube can be manufactured.
As mentioned earlier, the inside dimensions of the tube in its relaxed state must be smaller than the electrode assembly for this design to work. However, if the walls of the tube were straight when in their relaxed state, they would have a natural tendency to form an arch when compressed around the electrode assembly, making good surface contact difficult to maintain. To compensate for this, the tube walls must be fabricated with a predetermined inward curve in their relaxed state in order that they rest flat against the electrode assembly. This makes manufacturing the tube more complicated and expensive, and control of this curvature is difficult to maintain using an extrusion process.
It is noted that gas lasers are usually filled to a pressure significantly less than atmospheric pressure. Yet another drawback arises from the fact that this second design relies on the difference in pressure between the atmosphere and the gas mixture and the resulting compressive force on the tube walls to help squeeze the electrode assembly. However, this means that laser performance depends on the outside atmospheric pressure. Accordingly, the laser may not be suitable for high altitude or space applications.
This design also requires that both sides of the tube deform, making it difficult to maintain accurate registration between the resonator optical elements if attached to the ends of the tube and the electrode assembly inside.
Finally, special tooling is required to assemble the tube.
Accordingly, there remains a need for a laser structure that enables efficient and symmetric heat transfer, reliable construction and efficient manufacturing techniques.